The present disclosure relates to an image forming apparatus comprising a fixing unit configured to fix a toner image on a sheet.
An electrographic image forming apparatus, such as a copying machine or a printer, comprises a fixing device configured to fix a toner image on a sheet. The fixing device includes a first rotating member (for example, a roller or a belt) heated by a heating source and a second rotating member (for example, a roller or a belt) forming a fixing nip between the first rotating member and second rotating member by being brought into pressure contact with the first rotating member. When the sheet passes through the fixing nip, the toner image is fixed on the sheet by heating and pressing the toner image.
In the above-mentioned fixing device, there is a fixing device unitized as a “fixing unit”. The fixing unit is configured to be detachable to an apparatus main body of the image forming apparatus for the convenience of maintenance and part replacement.
In the above-mentioned fixing nip, the toner image may be fixed to various sheets with different thickness and dimensions. For instance, in a case where the toner image is fixed on a nonstandard sheet, such as an envelope or a thin paper, when a pressure (hereinafter, called as a “nip pressure”) of the fixing nip is the same as that of another case where the toner image is fixed on a standard sheet, there is fear that the sheet is crinkled. In recent years, particularly, the sheet is diversified according to speeding up and colorization of the image forming apparatus, it is difficult that one nip pressure satisfies conveying performance and fixing performance to various types of the sheets.
Then, various mechanisms for changing the nip pressure in accordance with types of the sheets are being developed. For instance, there is a fixing unit including a compression spring that brings the first and second rotating members into pressure contact with each other and a nip pressure changing member that moves the second rotating member in a direction in which an inter-shaft distance from the first rotating member increases. In this art, the nip pressure changing member is provided with a gear which meshes with a gear on the apparatus main body side. The gear on the apparatus main body side is connected with a driving source provided in the apparatus main body.
In the art described above, the nip pressure changing member is provided in the fixing unit, enlarging and complicating the fixing unit. Still further, because the nip pressure changing member is provided in the fixing unit, the fixing unit inevitably receives a great load generated in changing the nip pressure, possibly receiving an excessive load.
In the abovementioned art, a rotation of the driving source provided in the apparatus main body is transmitted to the nip pressure changing member via the gear on the apparatus main body side and the gear on the nip pressure changing member side. According to this, there is a possibility of causing a tooth skipping between the gear on the apparatus main body side and the gear on the nip pressure changing member side and another possibility of malfunction due to an excessive axial torque applied to the gear on the apparatus main body side and the gear on the nip pressure changing member side.
Still further, in the abovementioned art, it is unable to rotate the nip pressure changing member without driving the driving source and it is difficult to rotate the nip pressure changing member when the driving source is stopped.